Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 230219

Area Forecast Discussion
National Weather Service Honolulu HI
419 PM HST Tue Aug 22 2017

An upper level low northwest of the islands will continue to move
away from the islands as high pressure builds to the north. A more
stable atmosphere is expected to build over the islands, with
drier conditions than the last couple of days. The high will
bring moderate to locally breezy trade winds to the islands into
the weekend, with clouds and showers favoring windward and mauka
areas. Trade winds are forecast to relax by the end of the upcoming
weekend as high pressure weakens to the north. Muggy conditions
could return with the new week.


An upper level low is about 350 miles northwest of Kauai this
afternoon, and the low continues to move away from the main
Hawaiian islands. Models continue this movement as the low
dissipates over the next 72 hours or so. A more stable atmosphere
is already spreading over the islands. The afternoon soundings
showed near normal values for 500 mb and 700 mb temperatures and
precipitable water (PW). Lihue will has colder than normal 500 mb
temperatures and above normal PW, however conditions are trending
towards normal.

A thunderstorm developed over the leeward slopes of the Big Island
this afternoon, but is already dissipating by 330 pm. Expecting a
more stable airmass, so have left thunderstorms and heavy rain out
of the forecast starting tonight.

High pressure building to the north will maintain a moderate to
locally breezy trade wind pattern into the weekend. Over the
weekend, the high will begin to weaken, and that weakening trend
is expected to continue into the first half of the new week.
Monday will see dewpoints climb into the lower 70s. Combined with
light winds, muggy conditions are possible Monday and Tuesday.


Breezier and drier trade wind weather is expected through
Wednesday as 1) a low aloft NW of Kauai moves W; 2) high pressure
consolidates to the N; and 3) a drier air mass moves over the
islands from the E. A trend toward a typical trade wind weather
pattern means that AIRMET TANGO will remain posted for mechanical
turbulence downwind of the islands, especially since numerous
PIREPs contained reports of moderate low-level chop today.

Otherwise, despite the development of an isolated brief thunderstorm
over the leeward slope of Mauna Loa this afternoon, VFR conditions
are expected to prevail, and no other AIRMETs are expected. Brief
MVFR VIS/CIG in SHRA will affect primarily N thru E sections of
the islands as trade winds deliver randomly distributed showery
low clouds, and may affect terminals in Lihue and Hilo.


With trade winds on the increase, a Small Craft Advisory (SCA) is
currently in effect for the typically windier zones around Maui
County and the Big Island. The SCA is in effect through Thursday

An upper level low has been heading west away from the islands,
and its influence has been waning across the area. Isolated
thunderstorms will likely be confined to the offshore waters west
and northwest of the state through the afternoon and evening
hours. More stable and drier conditions are moving over the island
chain from the east.

East-facing shores should see a slight increase in surf heights
as the trade winds rebuild. A longer period east swell may also
arrive during the second half of the week as a result of tropical
cyclone activity in the eastern Pacific. South-facing shores may
see a slight increase in surf this weekend from a swell train that
originated from a Tasman Sea gale. A small background southeast
swell is also expected through the forecast period.


Small Craft Advisory until 6 PM HST Thursday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.



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